Effects Of Grafting On Physiological And Biochemical Index Of Cucumber Seedling And Root Exudates Under Copper Stress

Copper at low concentrations is an essential element for organisms because it is involved in a number of physiological processes,however, excessive levels of copper can cause a range of morphological and physiological disorders. Cucumber plant is considered as a Cu-sensitive crop. However, many plant species and types have higher tolerance or accumulation ability to copper. Therefore, one way to avoid or minimize copper toxicity would be to graft crops on to rootstocks capable of reducing the effects of external copper on the shoot.Greenhouse experiments were carried out to determine physiological and biochemical characteristics, and root exudates of cucumber plants(Cucumis sativus L. cv. ‘Xintaimici’) under copper stress, either ungrafted or grafted onto the rootstock(Cucurbita ficifolia). The main research results were summarized as follows:Excess Cu greatly inhibited the plant growth; however, this inhibition was significantly alleviated by grafting treatment. Under Cu stress, total chlorophyll, carotenoids, net photosynthesis(Pn), stomatal conductance(Gs) and intercellular CO2 concentration(Ci) of grafted plants were significantly higher than ungrafted plants.Cu concentration in leaves, stems and roots of cucumber seedlings was significantly raised under copper stress. Cu concentration in roots of grafted plants was significantly higher than ungrafted plants, however, Cu concentration in stems and leaves were obviously lower than ungrafted plants. This implies that grafted plants had higher ability to exclude Cu transport to shoots and leaves. Concentrations of N, K, Ca, Mg and Fe in leaves of grafted cucumber plants were greatly higher than those of ungrafted plants under Cu stress, thus the grafted cucumber plants maintained a better plant nutritional status.The accumulation of reactive oxygen species(ROS) significantly increased in cucumber roots and leaves under Cu stress, and resulted in lipid peroxidation, and the levels of ROS and lipid peroxidation were greatly decreased by grafting. Activities of protective enzymes and the contents of ascorbate and glutathione in leaves of grafted plants were significantly higher than those of ungrafted plants under Cu stress.Activities of H+-ATPase, H+-PPase and Ca2+-ATPase in PM and/or tonoplast in grafted were obviously higher than that in ungrafted plants, result in higher ability in grafted plants to promote Cu2+ into cytoplasm and areal distribution, to keep the low concentration of free Cu2+.Better performance of grafted cucumber plants were attributed to higher IAA, ABA contents and ratio of GA/ABA.Secretion of amino acids and organic acids were increased under copper stress. The amino acids secretions of grafted seedlings roots were obviously higher than ungrafted seedlings except for Phe and Val. At the same time, the secretion of oxalic acid, malic acid, acetic acid, citric acid, cinnamic acid, ρ-Hydroxybenzoic acid and benzoic acid of grafted seedlings were also significantly higher than ungrafted seedlings. Therefore, more Cu2+ were restricted in soil by chelating, complexing and precipitation with root exudates,and its toxicity was decreased.